Induction furnace and method of operating the same



Nov. 28, 1933.

INDUCTION w. E. MOORE 1,937,065

FURNACE AND METHOD OF OPERATING THE SAME 2 Sheets-Sheet 1 Filed Oct. 10, 1931 gwuewl oz w. E. MOORE 1,937,065

INDUCTION FURNACE AND METHOD OF OPERATING THE SAME Nov. 28, 1933.

Filed Oct. 10, 1931 2 Sheets-Sheet 2 Qwwntoz atented News 8, 1933 to Gory-crouch, Pltto= burgh, has

Applicotion no, Fierlol No, bfilhllltl ll @lolu r,

This invention relates to electric and more particularly to furnaces oi the induction type i Induction furnaces, as heretofore constructed,

have usually been one of three broad types, namely, (i) the all iron core furnace comprising a generally horizontal annulor chonnel which holds the molten metal and constitutes o secondary around the closed magnetic circuit; (2) m the so-called submerged loop iron core furnace, in which there is a U-shaped loop projecting from the crucible of molten metal through which the iron core extends, such core being excited by a primary inductor coil; and (it) the so-colled air core furnace in which the exciting winding or inductor coil surrounds the crucible or is so related thereto as to project the elternuting magnetic flux through the body of rnetel con tained therein.

The first of these types objectionable because or its extremely low power doctor; the foot that the annular secondary channel is oi refractory material end lilsely to hreoh moreover has n lorge heat radiating surface; the molten metal secondary in the annular channel is apt to rupture at its thinnest section due to magneto-strictlon or so-called biochcfiect, when large power and high amperage is epplied as, for instance, in rapid hosting; end that the p furnace cannot be started with o, cold chorge without first priming it with sufficient molten metal to form a secondary circuit,

The second or submerged loop type of furnace has the advantage of providing stotic hood of molten metal over the secondary loop, which prevents the loop from rupturing by pinch-= efiect". It has been popular for brass and bronze melting, but due to the difficulty oi obtoining suitable refractory. material oi which to construct the submerged loop so as to moire possible reasonable cost of maintenance, this iurnuce hos not been satisfactory for use in larger sizes or in melting metals requiring it, higher tempero= ture, such as nickel, chromium, iron or steel. The submerged loop type of furnace is also subject to the some objection as the horizontal ring type, insofar as the problem of cold starting is con= cerned.

The third or air core type or furnace in which the inductor winding surrounds the open top crucible, is objectionable because, on account or the exceedingly high magnetic reluctance, an abnormally heavy exciting current is required and the furnace works at an abnormally low power factor at ordinary commercial frequencies resulting in exceedingly high hee losses in the inductor and other windings. More over, the mechanical structure or the ilu'i'nace,

in which n coil surrounds the crucible, makes it clumsy end awkward to handle, and diidcult to tilt for the purpose of pourinc. To improve to some extent the power factor and efilciency of such furnaces, it has been proposed to operate them at very high frequencies, but for purpose, special generators or converters for @5 high frequency currents ore required, and such equipment is expensive, inefficient and trouble= some.

it hasbeen proposed to provide partial. iron core magnetic circuits in this last mentioned type of furnace in order to somewhot decrease the reluctance, confine and direct the exterior niag= netic flux, and increase the efficiency. These magnetic iron cores have been arranc'ed to project up in the crucible from the bottom, or 7 to extend down into the some from the top, but owing to the fact that they are inside oi. the crucible, such. arrangements are undesirable from o precticsl standpoint, since these cores interfere with the handling of the charge and become unduly heated.

The general object of the present invention is to provide on induction type furnace which sum be tree from most or the objections and diced yentoges of three previously lmown types and 35 their modifications above discussed, and which sheh operate reasonable efiiclency and at o relotively high power factor with alternating cur rents or moderate or or the usuol commercial frequencies, 99

One oi the specific objects of the invention is to provide oniuduction furnace which can be cold, is to say, which does not require priming with molten metal.

Another object is to provide on induction for once in which there are no refractory rings or loops employed; in which the heated refractory crucible maybe surrounded by heat insulating material to minimise heat losses, in accordance with usual practice in other types of furnaces; 1m and which rosy be enclosed in on iron or steel shell without incurring the usual stray electro magnetic losse r I A still further object is to provide a furnace in which the exciting coil or winding is mechanically 5, free from and unconnected with the crucible, so that the crucible may be conveniently handled, lil'ted or tilted, as desired.

Heretolore, in constructing induction furnaces oi the horizontal channel or submerged loop type, 1 1o V out modification of the crucible.

it has been necessary to design the channel or loop so that it will have the proper cross section relative to the power input, for the particular metal being melted, each of such metals having a definite specific conductivity. in order to handle metals of a different specific conductivity, it was necessary to redesign the channel or loop for each case.

Another object of the present invention, therefore, is to provide an induction furnace having means by which the reluctance of the magnetic circuit may be adjusted so as to compensate for the different conductivity of different metals being melted, where by the load on inductor coil furnace power supply may be regulated as required, with in other words, i propose to use same furnace for melting metals of widely different electrical characteristics without making anymechanical change in the structure of the furnace itself, merely by adjusting or varying the magnetic characteristics oi path through which the flux passes. i may ac complish the same purpose however by changing the voltage or intensity of the power supply.

In order that the invention, and the means for carrying the into effect, may be readily derstood, reference is had to the accompanying drawings, forming" part of this specification, and. illustrating more or less diagrammatically several different embodiments oi the invention. In these drawings:

Fig. 1 is a vertical section through one form of the improved furnace, parts being shown in clevation:

in Figure 1 looking in a direction at right angles to that of Figure l, the magnetic core being shown in section and parts being omitted for the sake of clearness;

Fig. 4 is a vertical section similar to Figure 1, showing one form of .a three-phase'furnace constructed in accordance with the invention; and

Fig. 5 is an inverted plan view showing a slightly different arrangement of three-phase furnace.

Referring to the drawings in detail, and more particularly first-to Figures 1 and 2 thereof, my improved furnace comprises a material holding chamber or crucible 1 constructed of suitable refractory material and of any convenient form such as cylindrical or slightly tapered mounted with its axis vertical. Surrounding the crucible 1 is a Jacket or a packing of suitable insulating ma.- terlal 2, to prevent excessive heat losses, and this insulating material and crucible may be enclosed in a metallic cage or shell 3, which may be made of iron orsteel. Trunnions 4 are preferably secured to the sides of the shell 3 by means of which the furnace may be lifted and tilted for the purpose of pouring the charge. {my other of the well known methods of mounting and tilting the furnace may, of course, be employed.

Projecting from the crucible is a relatively small and thin flat pocket or box 5 communicating at one end with the interior of the crucible and constituting a material melting chamber of the pot type. This pocket or box may project in any direction from the crucible, so long as it is below the normal level of the charge, but I have illustrated it as depending from the bottom of the crucible, as this has been found to be a convenient location. As shown in Figure 2, this pocket or box 5 may have flattened parallel sides. It exoevgoce tends down through the insulating material 2, and the shell 3 is cut away at 3 to avoid magnetic leakage, as hereinafter described. In order to assist in confining the heat in the pocket or box, i. preferably surround it with a layer or wrapping ii oi heat insulating material, such, for example, as asbestos.

E heat the furnace by inducing strong eddy currents in that portion only of the charge which is contained within tliepocket or box 5, and to this end, ll provide means for concentrating the magnetic flux upon and directing it transversely through this pocket. Thus, because of the comparatively narrow width of this pocket, the magnetic gap is short.

The means for doing this, as shown in Figure 1, comprises a laminated U-shaped core or frame 7 soft iron or other magnetic material. This frame is disposed in a vertical plane below the furnace, substantially centrally thereof. Proiecting inwardly from the upper end of each. leg (if the frame 7 toward the box is a laminated pole piece 8. These pole pieces may be interleaved v with the frame 7, as shown in Figure 2, or otherwise slidably connected therewith, so that they may be adjusted longitudinally toward and from each other. It will be seen that the frame 7 and pole pieces 8 constitute, in effect, a nearly closed magnetic core having an air gap between the adjacent ends oi the pole pieces, and that the pocket or box 5 lies within this air gap.

On the magnetic core above described, and preferably on the pole pieces 8 themselves, are wound inductor coils or exciting windings 9 and these are connected by conductors 10 with a source of alternating current of any usual commercial frequency. The coils are preferably shunted by means of a conductor 12 having therein a static condenser 11, in order to improve the power factor of the apparatus. A rotary or synchronous condenser may also be em ployed instead, if desired.

Means are provided for adjusting the pole pieces toward and away from the pocket or box 5 in order to vary the length of the air gap and hence the reluctance of the magnetic circuit. The means for doing this, as shown in Figure 1, may comprise screws 13 having a swivelled connection 14 with the respective pole pieces 8, and threaded through brackets 15 secured to a. fixed support, as for example, the frame 7 itself. These screws ,may have cranks formed upon their outer ends or, as shown in the drawings, the ends may be squared. as illustrated at 16, in order to receive a suitable key or wrench by which the screws may be turned. It is obvious that by turning these screws, the air gap may be adjusted, as required. Any other well known means for adjusting the pole pieces may be used.

.Qwlng to the fact that the magnetic core is laminated, it usually will not become excessively hot, and the pole pieces are shielded from the heat of the furnace both by the covering 6 and by the insulation 2. However, if necessary, provision may be made for cooling the magnetic core by means of an air blast or the like.

Similarly, the inductor coils 9, being exposed to the air and substantially shielded from the heat of the furnace, will, at least in the smaller sizes of furnace, not overheat. These coils, however, may be constructed of hollow tubing, if desired, through which cooling water may be circulated in a well known manner.

It will be observed that owing to the relatively low magnetic reluctance of the induction ap'patill) rpcaocs ratus, and the relatively small amount of mag= netic leakage, the power factor will be relatively high, even at commercial frequencies. it will also be appreciated that the concentration oi the magnetic flux through the pocket 5 by means of the pole pieces 8 permits the use of the steel shell 3 without materially interfering with the operation of the inductor coils, this shell being, however, preferably cut away as shown at 3 to minimize leakage of stray flux from the pole pieces.

It will be still further observed that the iur nace, as shown, is mechanically independent of the inductor coils and core and may be moved, lifted or tilted, as desired, without disturbing the induction apparatus. The induction apparatus may, however, in some cases, be attached to and movable with the furnace shell.

As explained in the preamble, it is usually neces-= sary to prime an induction furnace with molten metal in order to start it. In my improved con struction, however, this isunnecessary. in order to start the improved furnace, I simply place in the pocket 5 a slab or slug X of solid cold metal, as shown in dotted lines in Figure l. Thereupon, when the current is turned on, eddy currents will be induced in this slab or slug, of sumcient strength to melt the same, and the heat will then be imparted to the rest of the charge. I believe it is broadly new with me to start or to operate an induction furnace by concentrating the magnetic flux upon a small portion only of the charge, as distinguished from the entire mass or bulk thereof.

In melting metals of widelydiilering electrical conductivity, it is desirable to vary the relation of the magnetic circuit to the charge in order to regulate the load and prevent overheating oi the transformer. This may be done either by vary= ing the length of the air gap, by adjusting the pole pieces 8, as described, thus changing the reluctance of the magnetic circuit or by shifting the furnace or magnetic core relative to each other. In Figure 3, I have illustrated, in dotted lines, how the magnetic core 7 and t may be shifted either laterally or vertically, so as to vary its relation to the charge contained in the pocket 5. When the core is in full line position, as shown -in Fig. 3, the eddy currents induced in the metal in the pocket have all the space between the axis of the core and bottom of the pocket to circulate in. In other words, the sec ondary return circuit is of relatively large cross section and relatively low resistance. In this case, the currents flowing in such secondary, and, consequently, the power input to the furnace, are a maximum. As the magnetic core is shifted toward its lower or eccentric position indi= cated in dotted lines, it will be seen that the cross-section of this secondary return circuit be comes less and its efiective resistance consequently increases, and this results in a reduction in the absorption of power by the furnace.

Thus, when melting metals or higher electrical conductivity, such as copper, the magnetic core may be either shifted to increase its eccentricity, as shown in Figure 3, or the air gap must be increased, so as to avoid overloading. When melting material of lower conductivity, such as nickel or steel, the air gap may be smaller and the magnetic core may be substantially centered upon the pocket.

I believe it is broadly new with me to provide an induction furnace having means by which the induction characteristics can be varied in ac= cordance with the particular metal being treated, in order to regulate the load, as desired.

"While in Figures 1, 2 and 3, I have illustrated my improved furnace as of the single-phase type, it is clear that the same principle may be employed in connection with three-phase circuits.

in Figure d, l have illustrated conventionally one form of three-phase furnace comprising a crucible l having three pockets 5 5 and 5, depending from the bottom thereof. A laminated magnetic core l is provided having a plurality of branches l and 7, terminating in pole pieces b il and ii, having air gaps between them, in which air gaps the several pockets are disposed. 0n the core is is wound a three-phase winding 9 9 and 9 shown as connected in delta and supplied with current from a three-phase circuit ic it is not necessary, however, that the several pockets be parallel, as shown in Figure l. They may be disposed radially or otherwise.

In Figure 5, which is an inverted plan view, I have shown three pockets 5, projecting from the bottom of a furnace 3 and disposed radially about a common center. Associated with each pocket is a magnetic structure 8' which may be similar to the structure 7, 8, shown in Figure 1, and the-inductor coils or winding 9' on these magnetic cores are connected to a three-phase circuit 10', as shown. The shell is preferably cut away at 3 adjacent the cores, as in Fig. 1. Other arrangements for polyphase circuits may, of course, be employed, and it will be understood that in connection with such arrangements as shown in Figures l and 5, the crucibles and magnetic structures may be shifted, as shown in Fig-. ure 3, or means may be provided for varying the air gaps, as shown in Figure 1.

While I have shown the inductor or primary coils 9 on the laminated pole pieces 8, I find that on certain arrangements of my invention it is desirable to wind these coils on the bottom leg of the magnetic circuit 7 in Figure 1, in a manner similar to coils 9 9 9 in Figure l. At other times, ll may find it desirable to wind the exciting coils on the vertical legs connecting the portions 7 and ii in Figure l, or on l l and 7 in Figure 4. in still other cases I may find it desirable to wind the inductor coil over practica ly the entire laminated magnetic structure.

in certain arrangements of my invention I prefer to regulate the power input and rate of melting or temperature by regulating the power supply without changing the relation of the furnace and inductor elements as by means of an induction regulator for varying the voltage of the power supply or by means of an adjsutable reactor coil, or by means of a field rheostat on the generator supplying the furnace in the usual and well lniown way.

Uther arrangements of this apparatus not here shown will r adily be apparent to those skilled in art, as, for instance, the box 5 may project radially from the sides of the crucible in Figures l. and 4, or any angle thereto.

ll claim. is:

l. in an induction furnace, a crucible adapted to contain the material to be melted, a relatively thin pocket projecting from said crucible and closed at one end, and means for causing an alternating magnetic flux to traverse said pocket iron i. side to side in the direction of its smallest dimension.

2. Ln an induction furnace, at crucible adapted to contain the material to be melted, a relatively thin pocket projecting down from and below the bottom of said crucible and closed at its lower end, and means for causing an alternating magnetic flux to traverse said pocket from side to side in the direction of its smallest dimension.

3. In an induction furnace, a crucible adapted to contain the material to be melted, a relatively thin pocket projecting from said crucible and closed at one end, a core of magnetic material having opposed pole pieces, said pocket being disposed between said pole pieces, and means for setting up an alternating magnetic flux in said core.

4. In an induction furnace, a crucible adapted to contain the material to be melted, a relatively thin pocket projecting from said crucible and closed at one end, a core of magnetic material iorming a nearly closed magnetic circuit having an air gap, said pocket extending within said air gap, and an inductor coil surrounding said core.

5. A self-starting electric furnace of the induction type comprising a crucible having a closed pocket projecting down therefrom and adapted to receive a solid slug of the material to be melted, and means for inducing in such solid slug eddy currents of sumcient strength to melt the same.

6. The method of starting an electric induction furnace having a crucible provided with a relatively small pocket extending therefrom and communicating therewith which comprises placing in said pocket a solid slug of metal, and then subjecting such slug to the action of an alternating magnetic flux to induce therein currents of sumcient volume to melt the same.

7. The method of operating an electric induction furnace which comprises supporting a small portion of the charge in the form of a relatively thin mass in the solid state connected with the main body of the charge, and causing an alternating magnetic flux to pass transversely through said relatively thin mass from side to side thereof, whereby eddy currents are induced therein.

8. The method of regulating an electric induction furnace having an inductor coil and a core of magnetic material associated therewith for directing the flux through the charge, which method comprises shifting the relative position of the furnace and the core of magnetic material in a direction transverse to the axis of the said core to vary the volume of the currents induced in the charge.

ii. The method of regulating an electric induction furnace having an inductor coil and a core of'rnagnetic material associated therewith for directing the flux through the charge, which method comprises shifting the relative position of the furnace and the core of magnetic material in a direction transverse to the axis of the said core to vary the effective resistance of the secondary circuits in that portiton of the charge in which currents are induced.

it. An electric induction furnace comprising a material holding chamber of relatively large capacity, a pot-type material melting chamber of relatively small capacity projecting down from and located wholly below said material holding chamber and communicating at its upper end only therewith, and an inductive energizing coil adjacent but wholly outside of said material melting chamber. 1

ii. The method of operating an electric induction furnace which comprises separating a small portion of the metallic charge in the form of a relatively thin mass in contact at one end only with the main body of the charge, and causing an alternating magnetic flux to pass transversely through such separated relatively thin mass, whereby eddy currents are induced therein and sumcient heat generated to melt the entire charge.

WHLIAM ENOCH MOORE. 

